This invention is in the field of auto-focusing techniques and relates to a method and apparatus enabling focus error correction to maintain a desired position of an article.
There are a number of optical systems utilizing focus error correction techniques. Such optical systems, which typically comprise an illumination system equipped with suitable light directing optics and a detection system equipped with light collecting optics, are used for inspection/measurements of articles, reading/recording information in an information carrier, etc. The focus error correction technique ensures that the article under inspection (or an addressed layer of the information carrier) is located in a focal plane defined by the light directing optics.
According to one conventional auto-focusing technique disclosed, for example, in U.S. Pat. Nos. 4,123,652 and 5,563,702, a light beam scattered from an article is directed through anamorphic elements, and the shape of the light beam cross-section after passing these elements is monitored. The basic principle underlying the implementation is that a cylindrical lens produces astigmatism. A focused beam is first displaced from an in-focus point in a first direction, and then in a second direction perpendicular to the first direction. The beam cross section therefore changes from being oblong in the first direction, to being circular, to being oblong in the second direction. Thus, an auto-focus beam is typically split into a pair of sub-beams, which propagate along mutually perpendicular optical paths onto a pair of detectors through a pair of astigmatic elements, respectively. The detectors are quadrature photodiodes. When the article is in focus, both detectors are illuminated by a circular beam. As the article goes out of focus, the horizontal diameter of the beam decreases on one detector and increases on the other.
This changes the electrical output from the quadrature detectors and the focus correction signal is thus calculated accordingly.
There is a need in the art to facilitate focus error correction by providing a novel method and apparatus utilizing a dynamic auto-focusing technique. The present invention enables the detection of a position of the plane defined by an article under processing relative to the focal plane of an optical system, thereby enabling maintenance of accurate focusing of an incident beam from the optical system to the article.
The term xe2x80x9cprocessingxe2x80x9d used herein signifies any procedure applied to the article during which the article should be maintained in a preset plane. This procedure may be inspection, information reading/recording, etc.
One aspect of the present invention is based on the detection of at least two interference patterns formed by light components returned from an elongated illuminated region on the article and passing through at least two symmetrical peripheral regions of the optical axis of a focusing/collecting lens arrangement. The term xe2x80x9cperiphery regionsxe2x80x9d used herein signifies regions of light propagation along an optical axis of the lens making substantially large angles to and lying substantially far from the optical axis (as compared to paraxial regions of the optical axis).
Each of the interference patterns is created by interference between collected light formed of light components propagating within one of the periphery regions and collected light formed of light components propagating within the paraxial regions of the optical axis. The relation between the interference patterns is indicative of the phase difference between the collected light formed of light components propagating within the paraxial and periphery regions, and consequently, of the out-of-focus position of the illuminated region.
One concept underlying the present invention is based on the following known phenomena. Light waves returned from in-focus and out-of-focus planes and passed through a focusing lens have substantially flat and spherical wave fronts, respectively. Consequently, information indicative of distortions produced at any out-of-focus location is actually that contained in light propagating within a periphery region of the optical axis, rather than that associated with a paraxial area. Hence, it is desired to analyze light components propagating within the periphery regions, to detect the focus error. On the other hand, light components propagating within the paraxial and periphery regions of the optical axis have different optical paths, respectively, and therefore have a certain phase difference indicative of the out-of-focus position of the article.
In the inventive apparatus, the same focusing optics (typically composed of one or more objective lens) directs incident light onto an elongated region of the article, and collects light returned from the illuminated elongated region. This focusing optics presents an aperture stop whose physical dimensions define the collected light portion. Collected light, which contains information of the illuminated region and of the aperture stop, enters a focus detection unit, and is directed towards the sensing surface of a detector through an optical system.
When the collected light passes through the optical system, an image of the aperture stop is first created (by a first lens arrangement) in a plane conjugate to the aperture stop plane and extending in an X-Y plane perpendicular to the optical axis of light propagation through the optical system. Then, at least three spatially separated light components are picked up (selected) from the light indicative of this image by appropriately locating at least three slits in the image plane, and are imaged onto the sensing surface of the detector by suitable optics (second and third lens arrangements). The slits are accommodated such that at least one of the selected light components contains light propagated within the paraxial region of the optical axis of the focusing/collecting optics, and the at least two other light components contain light propagated within two symmetrical periphery regions of the optical axis. The slits are made in a blocking plate (located in the X-Y plane), extend along the X-axis (parallel to the elongated illuminated region), and are aligned in a spaced-apart parallel relationship along the Y-axis, such that the optical axis of the optical system intersects with an axis of the at least one central slit.
The central slit may be relatively long, extending across the entire blocking plate, or may be in the form of a two-part slit, the two parts being spaced apart from each other along the X-axis and being spaced from at least two side slits along the Y-axis. The side slitsmay be relatively short (as compared to the long central slit). The side slit(s) located at opposite sides of the central slit are centrally symmetrical relative to the intersection point between the slits"" plane and the optical axis of the optical system, and are spaced from each other along both the X- and Y-axes.
According to one embodiment of the invention, there are provided two side slits located at opposite sides of the central slit. In this case, two interference patterns are created.
According to another embodiment of the invention, two pairs of side slits are located at respective opposite sides of the central slit. In this manner, a dynamic range of the detected signals can be extended. The slits of each pair may be spaced from each other along the Y-axis only, in which case two interference patterns are created, or may be spaced from each other along both the X- and Y-axis, in which case two pairs of interference patterns are created.
The slits are sufficiently thin to cause the diffraction of light emerging from the slits. Two interference patterns are created on the sensing surface by the interference between, respectively, the diffracted light ensuing from the upper slit(s) and the respective half (or part) of the central slit, and the diffracted light ensuing from the lower slit(s) and the other half (or part) of the central slit.
When the article is in focus, the two patterns are spaced from each other only along the X-axis, namely each two corresponding xe2x80x9cdarkxe2x80x9d fringes and each two corresponding xe2x80x9clightxe2x80x9d fringes in the patterns are aligned in two lines, respectively. This is the so-called xe2x80x9cbest focus imagexe2x80x9d.
When the article goes out of focus, the optical paths of the collected light components change, and, consequently, a phase difference is created between the light components emerging from the central and upper slits, as well as between the central and the lower slits. This causes the interference pattern to move up and down on the sensing surface of the detector. The two imaged interference patterns are formed by light components propagating symmetrically with respect to the optical axis. Therefore, images corresponding to the before-focus and after-focus locations of the illuminated region (line) displace into two opposite directions, respectively, relative to the xe2x80x9cbest focus imagexe2x80x9d. In other words, when one interference pattern moves up, the other pattern moves down and vice versa. By estimating the phase difference between the two patterns, the focus error can be estimated.
By equipping the apparatus with a feedback loop, typically consisting of a suitable processor interconnected between an output circuit of the detector and a servomotor coupled to the article support stage or/and to the focusing optics, it is possible to perform focus error correction successfully.
There is thus provided, according to one aspect of the present invention, an optical apparatus for determining a position of an article, the apparatus comprising an illumination unit, focusing optics and a focus detection unit, wherein:
the illumination unit is operable to generate incident light and illuminate an elongated region of the article for producing light returned from the illuminated region;
the focusing optics directs the incident light toward the article and directs at least a portion of the returned light towards the focus detection unit; and
the focus detection unit comprises an optical system and a detector, the optical system being operable to collect the directed portion of the returned light and create at least two images on a sensing surface of the detector in the form of at least two interference patterns, respectively, wherein at least one pattern is created by interference between light components of the collected light that propagated within a first periphery region of an optical axis of the focusing optics and light components of the collected light that propagated within a paraxial region of said optical axis, and at least one other interference pattern is created by interference between light components of the collected light that propagated within a second periphery region of said optical axis, substantially symmetrical to said first periphery region with respect to said optical axis, and light components of the collected light that propagated within the paraxial region of said optical axis, data representative of a relation between intensity profiles in the at least two interference patterns being indicative of the position of the article relative to a focal plane of said focusing optics.
According to another aspect of the present invention, there is provided a focus error detection method comprising:
passing incident light through focusing optics and illuminating an elongated region, thereby producing light returned from the illuminated region;
collecting at least a portion of the light returned from said illuminated region and passed through said focusing optics;
picking up at least three spatially separated light components of the collected returned light, to cause diffraction of each of said light components and to allow interference between a central light component that propagated within a paraxial region of an optical axis of the focusing optics and at least one first light component that propagated within a first periphery region of said optical axis of the focusing optics, and interference between said central light component and at least one second light component of the collected returned light that propagated within a second periphery region of said optical axis of the focusing optics substantially symmetrical to said first periphery region with respect to said optical axis; and
creating at least two images in the form of at least two interference patterns, respectively, on a sensing surface of a detector, said at least two interference patterns being created by the interference of the separated light components, a relation between intensity profiles in the interference patterns being indicative of the position of the illuminated region relative to a focal plane of said focusing optics.
The present invention may be used with an optical inspection system, by simply adding a separate detection-collection means for collecting light returned from the illuminated region at elevation angles other than that collected by the focusing optics. There is no need for a separate illumination unit, though one may be provided if desired.
Thus, according to yet another aspect of the present invention, there is provided a system for an optical inspection of an article, comprising an optical apparatus for maintaining a desired position of the article, and at least one detection unit, wherein said optical apparatus comprises:
an illumination unit operable to generate incident light and illuminate an elongated region of the article for producing light returned from the illuminated region;
focusing optics that directs the incident light towards the article and directs at least a portion of the returned light towards a focus detection unit;
said focus detection unit comprising an optical system and a detector, the optical system being operable to collect the directed portion of the returned light and create at least two images on a sensing surface of the detector in the form of at least two interference patterns, respectively, wherein at least one pattern is created by interference between light components of the collected light that propagated within a first periphery region of an optical axis of the focusing optics and light components of the collected light that propagated within a paraxial region of said optical axis, and at least one other interference pattern is formed by interference between light components of the collected light that propagated within a second periphery region of said optical axis, substantially symmetrical to said first periphery region with respect to said optical axis, and light components of the collected light that propagated within the paraxial region of said optical axis of the focusing optics, data indicative of a relation between intensity profiles in the at least two interference patterns being indicative of the position of the article relative to a focal plane of the focusing optics; and
said at least one detection unit comprises a light collecting optics and a detection means, the light collecting optics being capable of collecting light returned from the article at elevation angles different from an elevation angle of collection of said at least portion of the returned light defined by said focusing optics.
According to yet another aspect of the present invention, there is provided a method of maintaining a desired position of an article during processing of the article, the method comprising:
(a) generating an incident beam and illuminating an elongated region of the article to produce light returned from the illuminated region;
(b) directing the incident light towards the article through a focusing optics, and collecting with said focusing optics at least a portion of the returned light and directing it towards a focus detection unit, said focusing optics defining an aperture stop of light collection;
(c) creating from the collected returned light at least two images of the illuminated region in the form of at least two interference patterns, respectively, said at least two interference patterns being formed by the interference between a central light component of the collected light that propagated within a paraxial region of an optical axis of the focusing optics and at least one first light component of the collected light that propagated within a first periphery region of the optical axis of the focusing optics, and interference between said central light component and at least one second light component of the collected light that propagated within a second periphery region of the optical axis of the focusing optics substantially symmetrical to said first periphery region with respect to said optical axis;
(d) detecting light indicative of said at least two images and generating data representative thereof, and
(e) analyzing the generated data to determine a relation between intensity profiles in the at least two interference patterns, and determining a relative position of the article with respect to a focal plane of the focusing optics, thereby enabling maintenance of the desired position of the article.
One embodiment of the present invention may be used with an optical inspection system for inspecting wafers, and is therefore described as such below, by way of an exemplary implementation. It is within the contemplation of the invention to provide other implementations as well, involving systems for inspecting other kinds of articles, including other nontranslucent articles, or reticles.